274  
FXUS10 KWNH 151847  
PMDHMD  
 
MODEL DIAGNOSTIC DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
146 PM EST FRI DEC 15 2017  
 
VALID DEC 15/1200 UTC THRU DEC 19/0000 UTC  
 
...SEE NOUS42 KWNO (ADMNFD) FOR THE STATUS OF THE UPPER AIR  
INGEST...  
 
12Z MODEL EVALUATION...WITH PRELIMINARY PREFERENCES AND CONFIDENCE  
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~  
 
...SHORTWAVE KICKING EAST TODAY FROM THE GREAT LAKES AND OHIO  
VALLEY AND BECOMING NEGATIVE TILT WITH OFFSHORE CYCLOGENESIS...  
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~  
PREFERENCE: NON-GFS BLEND  
CONFIDENCE: ABOVE AVERAGE  
 
MODELS HAVE A FAIR DEGREE OF SIMILARITY WITH MASS FIELDS ALOFT AS  
THE SHORTWAVE KICKS EAST OVER THE NEXT 24-48 HOURS...BUT STILL  
SHOW SOME SPREAD WITH THE DEEPENING RATE AND LOCATION OF THE  
SURFACE LOW. THE OVERWHELMING TREND OVER THE PAST FEW MODEL CYCLES  
HAS BEEN FOR A STRONGER LOW THAT IS POSITIONED A BIT FURTHER NW.  
FOR THIS REASON...WILL PREFER A BLEND OF THE 00Z ECMWF...00Z  
UKMET...AND 00Z ECMWF ENSEMBLE MEAN WHICH ARE GENERALLY SITUATED  
ON THAT END OF MODEL SPREAD. OVERALL THE DIFFERENCES ARE FAIRLY  
SMALL FOR THIS SYSTEM.  
 
18Z UPDATE: THE 12Z CMC HAS TRENDED CLOSER TO THE ECMWF...AND THE  
12Z ECMWF IS RELATIVELY CONSISTENT WITH ITS PREVIOUS RUN. THE 12Z  
GFS IS FURTHEST EAST WITH THE SURFACE LOW...SO THE CHANGE WILL BE  
TO A NON-GFS BLEND. DIFFERENCES WITH THIS SYSTEM REMAIN SMALL.  
 
   
..UPPER LEVEL LOW CURRENTLY OVER THE GULF OF CALIFORNIA
 
 
...OPENING INTO A WAVE AND EJECTING NORTHEAST INTO THE SOUTHERN  
PLAINS ON SATURDAY...DEAMPLIFYING ON SUNDAY AS IT PHASES INTO THE  
NORTHERN STREAM JET OVER THE OHIO VALLEY...  
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~  
PREFERENCE: NON-NAM BLEND  
CONFIDENCE: SLIGHTLY ABOVE AVERAGE  
 
THE 12Z NAM INITIALIZED ABOUT 20 METERS LOWER WITH ITS 500MB  
HEIGHTS OVER NRN MEXICO THAN THE SOUNDING FROM NEAR CHIHUAHUA  
(MMCU) OBSERVED THIS MORNING. THIS RESULTS IN A TROUGH THAT IS  
SLIGHTLY DEEPER AND MORE EXPANSIVE...AND EVENTUALLY LEADS TO A  
STRONGER SHORTWAVE THAT EJECTS NORTHEAST INTO THE PLAINS. THIS  
PRESENTED SOME DIFFERENCES RELATIVE TO THE OTHER MODELS WHICH  
TENDED TO BE MORE TIGHTLY CLUSTERED. ALTHOUGH OTHER SMALLER  
DIFFERENCES EXISTED BETWEEN THE GLOBAL MODELS...THEY WERE NOT  
CONSISTENT OR LARGE ENOUGH TO EXCLUDE. THEREFORE...THE PREFERENCE  
IS FOR A NON-NAM BLEND.  
 
18Z UPDATE: NO CHANGE TO PRELIMINARY PREFERENCE. MODEL SPREAD WAS  
REDUCED ON THE 12Z MODEL CYCLE WITH THE EXCEPTION OF THE NAM...SO  
THE PRELIMINARY PREFERENCE OF A NON-NAM BLEND IS MAINTAINED.  
 
   
..SHORTWAVE APPROACHING THE PACIFIC NORTHWEST TODAY
 
   
..DIGGING SHARPLY SOUTH ALONG THE WEST COAST ON SATURDAY
 
   
..POSSIBLY DEVELOPING INTO CLOSED LOW NEAR THE DESERT SOUTHWEST
 
 
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~  
PREFERENCE: 65 PERCENT 12Z ECMWF/UKMET  
35 PERCENT 12Z GFS/CMC  
CONFIDENCE: SLIGHTLY BELOW AVERAGE  
 
MODELS SHOW QUITE A BIT OF SPREAD OVER THE SOUTHWEST WITH THIS  
TROUGH...AND IT LARGELY RELATES TO IF AND HOW A CLOSED LOW FORMS  
ALOFT...AND (RELATED) HOW QUICKLY THE TROUGH EJECTS TO THE EAST.  
THE 12Z NAM SHOWS A SUBSTANTIALLY DIFFERENT SOLUTION WITH TWO  
DISTINCT WAVES OR VORT CENTERS...ONE FURTHER NORTH NEAR THE FOUR  
CORNERS AND THE OTHER MUCH FURTHER SOUTH NEAR THE SOUTHERN TIP OF  
BAJA CALIFORNIA. THE GLOBAL MODELS SHOW A PRIMARY VORT CENTER  
LYING SOMEWHERE IN BETWEEN AND GIVEN THE LACK OF SUPPORT...WILL  
EXCLUDE THE 12Z NAM FROM THE PREFERENCE.  
 
OTHERWISE...THE 00Z ECMWF AND 00Z ECMWF ENSEMBLE CLOSE OFF A LOW  
QUICKLY OVER NRN BAJA / FAR NW MEXICO AND ARE THE SLOWEST WITH  
EASTWARD PROGRESSION. SOME ECMWF ENSEMBLE MEMBERS EVEN BEGIN TO  
CUT OFF THE LOW AND LINGER IT IN THE VICINITY OF AZ/NM INTO THE  
MEDIUM RANGE PORTION OF THE FORECAST. THE TREND ON ALL THE GLOBAL  
MODELS HAS GENERALLY BEEN TOWARD A SLOWER PROGRESSION...SO THIS  
DOES LEND CREDENCE TO THE IDEA OF A CLOSED LOW DEVELOPING ALOFT  
OVER THE SOUTHWEST. THE TOP FIVE 500MB ANALOGS FOR THE SHORTWAVE  
AS IT DIVES SOUTH ALONG THE WEST COAST PER CIPS GUIDANCE (00Z  
CYCLE) ALL SHOW A CLOSED LOW DEVELOPING IN A SIMILAR AREA (AND  
SOME EVEN RETROGRADING SLIGHTLY). THE 12Z GFS SLOWED DOWN SLIGHTLY  
AND IS NOW FAIRLY SIMILAR IN TIMING TO THE 00Z UKMET AND 00Z CMC.  
GIVEN THE RELATIVELY GOOD AGREEMENT...THIS SEEMS LIKE A REASONABLE  
SOLUTION. HOWEVER THE PATTERN...SOME ENSEMBLE MEMBERS...AND  
ANALOGS SUGGEST THAT THE ECMWF ALSO CANNOT BE DISCOUNTED. FOR THIS  
REASON...A NON-NAM BLEND IS THE PREFERENCE FOR NOW.  
 
18Z UPDATE: GREATER WEIGHT PLACED ON THE 12Z ECMWF/UKMET  
SOLUTIONS. THERE CONTINUES TO BE A CONSIDERABLE AMOUNT OF SPREAD  
WITH HOW THE MODELS HANDLE THE TROUGH BEGINNING ABOUT 24-36 HOURS  
FROM NOW. THE 12Z ECMWF CONTINUED TO SHOW A CLOSED LOW...AND EVEN  
SHOWED A DEEPER SOLUTION AND A STALLED MOTION RELATIVE TO THE  
PREVIOUS MODEL CYCLE. THE 12Z UKMET HAS JOINED THIS IDEA NOW WHILE  
THE 12Z CMC HAS REMAINED RELATIVELY UNCHANGED. THIS LEADS TO A  
DIVERGENCE BETWEEN THE 12Z ECMWF/UKMET CAMP AND THE 12Z GFS/CMC  
CAMP. AS MENTIONED IN THE PRELIMINARY DISCUSSION...WITH A  
SHORTWAVE DIVING SHARPLY SOUTH...WOULD EXPECT A LOW TO CLOSE OFF  
AND TO HAVE A GREATER LIKELIHOOD FOR SLOWER MOTION.  
THEREFORE...WILL ADJUST THE PREFERENCE TO PLACE A GREATER WEIGHT  
ON THE SLOWER SOLUTIONS NOW.  
 
...NORTHERN STREAM PORTION OF THE SHORTWAVE APPROACHING THE  
PACIFIC NORTHWEST TODAY SHIFTING TO THE GREAT LAKES BY MONDAY...   
..SOUTHWARD SHIFT IN THE JET OVER THE NORTHWEST
 
 
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~  
PREFERENCE: NON-NAM BLEND  
CONFIDENCE: SLIGHTLY ABOVE AVERAGE  
 
AS THE 12Z NAM SPLITS THE TROUGH IN THE WEST...IT HAS A STRONGER  
VORT MAX SITUATED AT HIGHER LATITUDES -- IN CLOSER PROXIMITY TO  
THE NORTHERN STREAM JET. AS A RESULT...ITS DEPICTION OF THE  
EVENTUAL TROUGH OVER THE NORTHERN PLAINS AND GREAT LAKES IS MUCH  
DEEPER AND STRONGER THAN THE OTHER MODELS. OTHERWISE...THE OTHER  
GLOBAL MODELS ARE IN FAIRLY GOOD AGREEMENT ON THE TIMING AND  
STRENGTH OF THE INITIAL LOW AMPLITUDE WAVE AND SHOW SIMILARITY AS  
THE NORTHERN STREAM JET BUILDS SOUTH INTO THE NORTHWESTERN CONUS.  
THEREFORE A NON-NAM BLEND IS PREFERRED.  
 
18Z UPDATE: NO CHANGE TO THE PRELIMINARY PREFERENCE.  
 
MODEL TRENDS AT WWW.WPC.NCEP.NOAA.GOV/HTML/MODEL2.SHTML  
500 MB FORECASTS AT WWW.WPC.NCEP.NOAA.GOV/H5PREF/H5PREF.SHTML  
 
LAMERS  
 

 
 
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